The present invention is directed to the field of apparatuses for the formation of large-diameter ducts, pipes, conduits or the like, generally cylindrical in shape, from an extruded thermoplastic strip. U.S. Pat. No. 4,033,808 to Petzetakis discloses an apparatus for making large-diameter pipe from thermoplastic resin, the disclosure of which is hereby incorporated by reference. A synthetic-resin, hollow-profile strip is extruded and coiled helically around a winding drum to make a continuous tube. Adjacent turns of the helix are thermally fused together with axial pressure being applied on the strip so as to make a liquid-tight tube which can be used for a number of applications, particularly for subterranean use, e.g. as a waste or sewer conduit, storm sewer, or any other liquid conveyance.
The previous apparatus includes a drum for forming the conduit therearound, and the drum includes a core tube surrounded by a plurality of rollers upon which the extruded thermoplastic rolls as it rotates around the drum. The drum of the previous apparatus is fixed in diameter, and so only one diameter of pipe may be produced with each apparatus. Since a typical manufacturer of pipe manufactures conduit or pipe of several different diameters, it would be necessary to purchase and maintain several such apparatuses, which would involve considerable investment and ongoing expense.
In a conventional conduit-making process, "residence time" is the period in which plastic remains on the drum during the manufacturing process. Since the rate of manufacture is limited by residence time, it would be desirable to shorten this interval by increasing the drum speed. Typical manufacturing rates can range from 300 to 3500 lbs./hr. depending on the size of the pipe. Usually, large-diameter conduit or pipe is manufactured at about 1000-2000 lbs./hr. As the rate is increased, however, the conduit may not be completely cooled when coming off the end of the drum. This makes the conduit shrink slightly inward, undesirably decreasing the final conduit diameter by a small, yet significant, amount.